1. Field of the Invention
This invention relates to a process for making films of Tl--Pb--Sr--Ca--Cu--O compositions which are superconducting. The invention also relates to a method for fabricating microwave and other electronic devices from these films.
2. Background
Recent advances in the elevation of superconducting transition temperature of various oxide materials have provided the opportunity for applications of such materials in radiofrequency, microwave and other electronic technologies. Considerable progress has been made in a number of fabrication technologies related to forming these oxide superconductors into various electronic devices. The higher the transition temperature of the superconducting oxide, the more likely that material will be of value in such applications. Subramanian, U.S. Pat. No. 4,894,361, and Subramanian et al., Science 242, 249 (1988) disclose superconducting compositions of Tl--Pb--Sr--Ca--Cu--O. Two superconducting phases, one with a c-axis unit cell dimension of about 12 Angstroms (1.2 nm) and a superconductivity transition temperature (T.sub.c) of about 85 K, and one with a c-axis unit cell dimension of about 15 Angstroms (1.5 nm) and a T.sub.c of about 122 K, were identified. Methods for producing powders of these materials and single crystals of the higher T.sub.c material are also disclosed.
For most present electronic device applications, such as radiofrequency and microwave technology, thin films are proving to be the most useful form of superconducting oxide. Various methods of producing superconducting films from oxides materials have been described. See, for example, Venkatesan, SPIE Proceedings Vol. 1187 (1989). These methods can be described as either in-situ or ex-situ fabrication routes, depending on whether the superconducting oxide film is made during a single-step process encompassing deposition, reaction and crystalline growth of the desired superconducting phases or by a two step process involving the deposition of a precursor, followed by a second distinct reaction and crystalline growth step. The second distinct step has generally been described as annealing. See, for example, Laubacher et al., IEEE Trans. Magn., MAG-27, 1418, (1991).
In the case of yttrium barium copper oxide films this second step has generally involved the reaction of the yttrium, barium, copper and oxygen components above the orthorhombic to tetragonal phase transition temperature followed by further reaction of the nonsuperconducting tetragonal phase with oxygen near the phase transition temperature to form the superconducting orthorhombic phase. In-situ methods have also been developed for forming yttrium barium copper oxide films. These processes have generally involved either laser ablation or sputtering deposition in the presence of an oxygen atmosphere while raising the temperature of the substrates and the films being deposited to a temperature favoring reaction and crystalline growth of the superconducting oxide film.
In the case of other superconducting oxides such as the Tl--Ba--Ca--Cu--O materials, only ex-situ processes have been reported to be successful. Films consisting of Tl and/or Ba--Ca--Cu--O have been deposited, and the resulting films have been annealed in containers containing the volatile thallium oxide at an elevated temperature which favors the growth of the desired superconducting oxide phase. See, for example, Lee et al., Appl. Phys. Lett. 53, 329 (1988), Holstein et al., IEEE Trans. Magn., MAG-27, 1568, (1991), and Nabatame et al. Jap. J. Appl. Phys. 29, L1813 (1990).
Improved superconductor oxide film properties are necessary for the integration of these materials into electronic devices. Measurements of electrical transport properties and magnetic measurements on fabricated forms of superconducting oxides provide an estimate of their performance in electronic devices. An eddy-current measurement technique described by Doss et al., Supercond. Sci. Technol. 2, 63 (1989), has proven to be useful in evaluating superconducting films. Radiofrequency and microwave cavity resonator measurements provide an estimate of a superconductor oxide film performance for various passive and active devices as described in Portis et al. J. Superconductivity 3, 297 (1990). From measurements of X", the complex part of the ac susceptibility, of a superconducting oxide a relationship of magnetic flux lattice pinning with respect to temperature can be derived which is called an irreversibility line. See, for example, Y. Yeshurun and A. P. Malozemoff, Phys. Ref. Lett. 60, 2202 (1988) and R. B. Flippen and T. R. Askew, J. Appl. Phys. 67, 4515 (1990). The X" peak is the value of maximum adsorption of energy by the flux lattice. These X" peak values define the irreversibility line which is the boundary between the magnetic flux-pinned and flux-mobile regions in a superconductor exposed to an external magnetic field. For magnetic fields higher than that of the irreversibility line, magnetic flux is mobile in the superconductor and the critical current is zero. For magnetic fields lower than that of the irreversibility line, magnetic flux is pinned and a superconducting current can exist.
Various electronic devices fabricated from superconducting oxides have been reported. Problems with high contact resistance between metal and superconductor interfaces, poor superconducting surface properties, uncontrolled reactivity of superconducting oxide surfaces with conventional photolithographic chemicals and incompatibility with conventional lithographic techniques have limited the fabrication and performance of electronic devices produced from superconducting oxide materials. Some progress has been made on passive microwave device fabrication and performance using superconducting oxide films. See, for example, Lyons and Withers, Microwave Journal, 33, 85,(1990) and Withers et al., Solid State Technology, 33, 83, (1990).